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26/3/2020: séminaire d'Aurélien Ribes (CNRM)

Narrowing uncertainty on past and future human-induced warming using Kriging
Quand ? Le 26/03/2020,
de 10:00 à 12:00
Où ? Salle de réunion du LMD, T45-55, 3e étage
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Téléphone 0144272799
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Many studies have sought to constrain climate projections on the basis of recent observations. Until recently, however, these constraints had limited impact, and projected warming ranges were driven primarily by model outputs.

In this talk, we use the newly available CMIP6 climate model ensemble, improved observations, and a new statistical method adapted from Kriging to narrow uncertainty on past and future warming. First, we present this method and its inputs, in particular the model and observation error covariances. Evaluation of our method using a cross-validation procedure suggests that the method produces robust results and is not over-confident. Then, we derive consistent observationally-constrained estimates of attributable warming to date, attributable warming rate, the response to a range of future scenarios, the Transient Climate Response (TCR), and the Equilibrium Climate Sensitivity (ECS). The externally forced warming to date is estimated at 1.19±.15°C (2019 wrt 1850-1900). We find that historical observations narrow uncertainty on late 21st-century projected warming by about 50%, with both the upper and the lower bounds affected. Constrained warming in 2100 is 3.1±0.6°C in response to a moderate-low emission scenario (SSP2-4.5), and 5.2±1.0°C in response to a high emissions scenario, SSP5-8.5), while TCR is estimated at 1.44-2.35°C. A comparison of the results obtained with the CMIP5 and CMIP6 ensembles suggests discernably different behaviors and will be discussed. Overall, our results suggest that using an unconstrained multi-model ensemble is no longer the best choice for climate change projections, and that historical observations are consistent with higher warming than previously thought.

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